Conventional level measurement implements time domain reflectometry (TDR) by transmitting microwave or ultrasound pulses (reference pulses) from an antenna or waveguide to the surface (or interface) of a material in a container (or tank) and measuring signals reflected (echo signals) including signals from that surface or interface. For example, when a radar reference pulse reaches a material with a different dielectric constant, part of the microwave energy is reflected back which is received by a receiver as an echo pulse. The echo pulse has an associated echo amplitude. Generally, the echo pulse will have the same shape as the reference pulse that is sent down the waveguide, but its sign and magnitude depend on the change in impedance level. Known radar level measurement methods include non-contact radar typically being pulsed radar and contact radar typically being frequency modulated continuous wave (FMCW) radar.
For TDR-based level measurement devices or systems, the reference pulse is superimposed on the echo signal reflected by an object or interface in an at least partially filled tank, whose distance (or level) is to be measured. In known methods, in order to determine the time position of the echo pulse in the echo signal to determine travel time T to enable calculation of a level value, the time profile of the echo signal is compared with a stored reference echo signal that was generated by the level measurement system without any objects or product material in the measurement path or an otherwise empty tank. The echo signal generated for an at least partially filled tank is different from the reference echo signal generated for an empty tank by at least one additional pulse sometimes referred to as an interface pulse that results from the reflection of the reference pulse at the interface(s) in the tank (e.g., an interface between a product liquid and the gas above). The time position determination of the interface pulse requires comparing the reference echo signal with the echo signal, with the simplest case being the reference echo signal subtracted from the echo signal performed incrementally through subtraction of amplitude values that are located at corresponding time positions of the signal profiles of the reference echo signal and the echo signal.
Due to the change in impedance at the reflection surface (e.g., the interface), points on the echo signal curve including the interface pulse generally have an amplitude that deviates from the points on the reference echo signal curve. Making the comparison of the echo signal and the reference echo signal to determine the travel time T can thus become difficult since significant amplitude offsets between the echo signal and the reference echo signal are generally present which can lead to errors in the level measurement.